In the radial turbine which is typically employed as an exhaust gas turbine of a turbosupercharger, it is often desirable to be able to supercharge the engine even at a relatively low engine speed and this can be achieved by increasing the velocity of the exhaust gas entering the turbine by restricting the cross section of the passage leading to the turbine wheel. However, when the passage is restricted, the inlet pressure of the turbine or the back pressure of the engine exhaust system rises and the efficiency of the engine drops.
Therefore, if a plurality of moveable vanes are arranged in a throat located adjacent the outer circumference of a turbine wheel in a circle so that the area of the nozzles defined between the moveable vanes may be varied by rotating the moveable vanes over a certain angle as described in Japanese Patent Publication No. 38-7653, it is possible to ensure supercharging of the engine in a low speed range of the engine and, at the same time, to keep the back pressure of the engine exhaust system to a low level in a medium to high speed range of the engine.
However, in this structure, since the vanes are placed in the region where the velocity of the fluid flow is relatively high, the resistance loss of the fluid tends to be great and, therefore, the efficiency of the turbine tends to drop. And, since the nozzles are defined between neighboring moveable vanes, a slight error in the angle of the vanes causes a significant change in the opening area of the nozzles particularly when the opening area of the nozzles is small and, therefore, a difficulty arises in ensuring a control precision. When this turbine is applied as an exhaust turbine in a turbosupercharger, since these vanes are exposed to an exhaust gas flow of high temperature, it is difficult to adjust these vanes with a high level of reliability.
Furthermore, since the vanes defining the variable nozzles are arranged to rotate along and between the inner surface of the turbine casing and the inner surface of the back plate which closes the turbine wheel bearing end of the casing and the thermal expansion of the turbine casing having a complicated shape is highly uneven, the gap control of the variable nozzles particularly in a high temperature environment could be a serious problem.
As disclosed in Japanese Patent Laid Open Publication 53-136113, it is previously known to change the cross sectional area of a scroll passage of a turbine casing by providing a moveable flap which defines a part of the wall of the scroll passage. In this variable nozzle structure, the velocity of fluid flowing into the turbine wheel can be adjusted without involving much resistance loss with a simple structure, but the range of variation is not as wide as desired and particularly when the flap opening is great the flow heading toward the turbine wheel tends to be disturbed with the result that the distribution of the fluid velocity tends to be uneven and, therefore, the efficiency of the turbine drops.